1. Technical Field
The present invention relates to a logging system for sensing and determining properties of earth formations; and more particularly to a logging system using fiber optics for sensing and transmitting data containing information about a borehole of an oil well with a sonde.
2. Description of the Prior Art
FIG. 1 shows a sonic sonde that is known in the art, and used in a logging system for sensing and determining the rock formations in the borehole of the well or the like. The sonde has a transmitter (i.e. source) for providing a transmitter acoustic signal, and has receivers for responding to the transmitter acoustic signal, for providing electrical receiver signals containing information about rock formations in the borehole.
In operation, the transmitter emits a sound pulse, and its arrival at the receivers is recorded by recording apparatus that is not shown in FIG. 1 but is well known in the art. This process is repeated at a fixed rate as the tool traverses through the borehole. Because the p-wave velocity is faster in the formation than in the borehole fluid, the first arrivals are the waves refracted in the formation at the side of the borehole wall. The receiver spacing and the p-wave velocity in the formation determine the delay between the arrival times. The delays or quantities derived from the delays are plotted as a function of borehole depth. See also U.S. Pat. No. 4,162,400, hereby incorporated by reference, which describes a fiber optic well logging tool for passing through a borehole of an oil well. See also Well Logging For Earth Scientists, Chapters 15-17, Darwin V. Ellis, Elsevier, 1987, as well as Acoustic Waves in Boreholes, Frederick L. Paillet and Chuen Hon Cheng, CRC Press, 1991, both also hereby incorporated by reference.
The acoustic receivers are made of pressure transducers. Piezoelectric or magnetostrictive materials are used to sense the pressure signal. The voltage output of the piezoelectric or the current output of the magnetostrictive varies with the pressure. Sensitive electronics built into the sonde measures the voltage or the current. The measurements are processed and sent uphole via an electrical wireline.
FIG. 1 shows an idealized situation where the sonde is in the middle of a borehole that is ideally very smooth. All current tools use an extra transmitter located on the opposite side of the receivers. Sound pulses are emitted alternately from the transmitters. The measured delays from the two oppositely traveling refracted waves are averaged. The averaged delays are less sensitive to the more irregular contour of a real borehole as well as tool tilting.
The sonde described above record only the time delay between the arrival pulses. The receiver readings as a function of time after the transmitter pulse (sonic traces) are not used, and therefore, not recorded. The only data sent through the wireline are the delays. Even at fast logging speeds, the data volume is moderate and can be transmitted through the standard 7-wire wireline whose capacity is 1 Mbit/s.
Due to the advance in processing, interpretation, and modeling, more properties of the formation, borehole, and tool performance can be obtained by using the entire receiver traces in both receivers. A 10-millisecond trace at 5 microsecond intervals consists of two thousand data points. In addition, more than two receivers are used for array processing. The data collected at each logging depth consist of thousands of measurements (numbers) as opposed to the time delay measurement (a single number). The huge data volume generated from the newer acoustic logging tools severely limits the logging speed.
Fiber optic telemetry systems have been proposed to replace the standard 7-wire cable. U.S. Pat. No. 5,495,547, hereby incorporated by reference, describes a combination fiber-optic/electrical conductor well logging cable. The combination logging cable preserves all the functionality and structural integrity of the standard 7-wire plus one or more fiber wires integrated in. Fibers provide much higher bandwidth, allowing a much faster data transmission rate. Opto-electronics are used to convert electrical signals to optical signals fed into the fiber. The receiver electronics are the same as the traditional wireline acoustic tool. The use of fiber solves the problem of data transmission capacity. The much higher data rate is achieved by using opto-electronics downhole. This increases the cost of the tool and its rate of failure.
One disadvantage of the known designs is that the sensitive electronic are disposed in the sonde and subject to very harsh conditions, including high temperatures and undesirable vibrations when passing through the borehole. These adverse conditions can limit accuracies in data measurement, as well as accelerated electronic failure.